The blood vessel lining known as the endothelium is capable of secreting substances which allow blood vessels to dilate in response to several stimuli. One factor is called endothelium derived relaxant factor (EDRF). Acetylcholine is able to promote release of EDRF from blood vessels and when endothelium is damaged, then the response to acetylcholine is diminished. A number of patients, especially women with normal coronary arteries, suffer anginal chest pain due to abnormalities of the small blood vessels in the heart. To investigate whether these abnormalities are due to dysfunctional endothelium, we infused acetylcholine which relaxes blood vessels only in patients with normal endothelium and nitroprusside which relaxes blood vessels independent of endothelial function. Approximately 40% of patients with angina despite normal coronary arteries had demonstrable endothelial dysfunction of the small blood vessels in their heart. This occurred in equal proportion of patients with and without high blood pressure.